Point-contact/tunneling spectroscopy studies of the upper critical field in superconductors

Point-contact or tunneling spectroscopy has been performed on different typ es of superconductors in magnetic fields along with the magnetotransport me asurements on the same systems to determine the upper critical field B-c2. Namely, we studied the superconducting misfit layer compounds (BiSe)(1.10)( NbSe2) (with the superconducting transition T-c = 2.36 K, and a small aniso tropy in the superconducting properties), (LaSe)(1.14)(NbSe2)(2) (an extrem ely anisotropic crystal with T-c = 5.43 K), and also two members of the cub ic perovskite oxides - BaPb(x)Bi(1-x)0(3) (T-c = 10 K) and Ba1-xKxBiO3 (T-c = 23 K). The temperature dependence of the upper critical field obtained s pectroscopically can be satisfactorily described by the classical Werthamme r-Helfand-Hohenberg model for type-II superconductors in all above mentione d systems. This is in contradiction with the magnetotransport determination of B-c2 in (LaSe)(1.14)(NbSe2)(2) and Ba1-xKxBiO3 where unusual increase o f B-c2 was found at low temperatures. Possible explanation of this discrepa ncy is discussed in the framework of vortex melting in these two systems. I n other two systems the both methods give the identical results.